Cathode active material for lithium secondary battery and lithium secondary battery including the same

ABSTRACT

A cathode active material for a lithium secondary battery includes a lithium-transition metal composite oxide particle having a lattice strain (η) of 0.18 or less, which is calculated by applying Williamson-Hall method defined by Equation 1 to XRD peaks measured through XRD analysis, and having an XRD peak intensity ratio of 8.9% or less, which is defined by Equation 2. By controlling the lattice strain and XRD peak intensity ratio of the lithium-transition metal composite oxide particle, a lithium secondary battery with improved life-span characteristics as well as output characteristics is provided.

CROSS-REFERENCE TO RELATED APPLICATION AND CLAIM OF PRIORITY

The application is a continuation of U.S. patent application Ser. No.17/401,091 filed on Aug. 12, 2021, which claims priority to KoreanPatent Application No. 10-2020-0101564 filed on Aug. 13, 2020 and KoreanPatent Application No. 10-2020-0150764 filed on Nov. 12, 2020 in theKorean Intellectual Property Office (KIPO), the entire disclosure ofwhich is incorporated by reference herein.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a cathode active material for a lithiumsecondary battery and a lithium secondary battery including the same,and more particularly, to a cathode active material for a lithiumsecondary battery including a lithium-transition metal composite oxideand a lithium secondary battery including the same.

2. Description of the Related Art

A secondary battery which can be charged and discharged repeatedly hasbeen widely employed as a power source of a mobile electronic devicesuch as a camcorder, a mobile phone, a laptop computer, etc., accordingto developments of information and display technologies. Recently, abattery pack including the secondary battery is being developed andapplied as a power source of an eco-friendly vehicle such as a hybridautomobile.

The secondary battery includes, e.g., a lithium secondary battery, anickel-cadmium battery, a nickel-hydrogen battery, etc. The lithiumsecondary battery is highlighted due to high operational voltage andenergy density per unit weight, a high charging rate, a compactdimension, etc.

For example, the lithium secondary battery may include an electrodeassembly including a cathode, an anode and a separation layer(separator), and an electrolyte immersing the electrode assembly. Thelithium secondary battery may further include an outer case having,e.g., a pouch shape.

A lithium metal oxide may be used as a cathode active material of thelithium secondary battery preferably having high capacity, power andlife-span. However, if the lithium metal oxide is designed to have ahigh-power composition, thermal and mechanical stability of the lithiumsecondary battery may be degraded to also deteriorate life-span propertyand operational reliability.

For example, Korean Publication of Patent Application No.10-2017-0093085 discloses a cathode active material including atransition metal compound and an ion adsorbing binder, which may notprovide sufficient life-span and stability.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided acathode active material for a lithium secondary battery having improvedoperational stability and reliability.

According to an aspect of the present invention, there is provided alithium secondary battery including a cathode active material withimproved operational stability and reliability.

According to exemplary embodiments, a cathode active material for alithium secondary battery includes a lithium-transition metal compositeoxide particle. A lattice strain (n) calculated by applyingWilliamson-Hall method defined by Equation 1 to XRD peaks measuredthrough X-ray diffraction (XRD) analysis is 0.18 or less, and an XRDpeak intensity ratio defined by Equation 2 is 8.9% or less:

β cos θ=η sin θ+λ/D  [Equation 1]

in Equation 1, β represents full width at half maximum (FWHM) (rad) ofthe corresponding peak acquired through the XRD analysis, θ represents adiffraction angle (rad), η represents a lattice strain (dimensionlessnumber), λ represents anX-ray wavelength (Å), and D represents acrystallite size (Å).

XRD peak intensity ratio (%)=100×I(110)/{I(110)+I(003)}  [Equation 2]

in Equation 2, I(110) represents a maximum height of the peak of a (110)plane of the lithium-transition metal composite oxide particle by theXRD analysis, and I(003) represents a maximum height of the peak of a(003) plane of the lithium-transition metal composite oxide particle bythe XRD analysis.

In some embodiments, the lattice strain of the lithium-transition metalcomposite oxide particle may be a slope of a straight line, and thestraight line may be obtained by acquiring full widths at half maximumof all peaks appearing through the XRD analysis, and substituting theacquired full widths at half maximum in Equation 1 when plotting ahorizontal axis with sine and a vertical axis with β cos θ.

In some embodiments, the XRD peak intensity ratio of thelithium-transition metal composite oxide particl is in a range from 4 to8.9%.

In some embodiments, the lithium-transition metal composite oxideparticle may have a polycrystalline structure in crystallography.

In some embodiments, the lithium-transition metal composite oxideparticle may have at least one of a single particle, a primary particle,and a secondary particle in morphology.

In some embodiments, the lithium-transition metal composite oxideparticle may have a composition represented by Chemical Formula 1 below:

Li_(x)Ni_(1−y)M_(y)O_(2+z)  [Chemical Formula 1]

(in Chemical Formula 1, x and y are in a range of 0.9≤x≤1.2, and0≤y≤0.7, and z is in a range of −0.1≤z≤0.1, M is at least one elementselected from the group consisting of Na, Mg, Ca, Y, Ti, Zr, Hf, V, Nb,Ta, Cr, Mo, N, Mn, Co, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn and Zr).

In some embodiments, in Chemical Formula 1, (1−y) may be 0.8 or more.

In some embodiments, in Chemical Formula 1, M may include Co and Mn.

In some embodiments, the lithium-transition metal composite oxideparticles may be manufactured by reacting a transition metal precursorwith a lithium precursor.

In some embodiments, the transition metal precursor may be a Ni—Co—Mnprecursor.

In some embodiments, the lithium precursor may include lithiumcarbonate, lithium nitrate, lithium acetate, lithium oxide or lithiumhydroxide.

According to another aspect of the present invention, a lithiumsecondary battery includes a cathode which includes a cathode activematerial layer including the cathode active material according to theabove-described embodiments; an anode facing the cathode.

The lithium secondary battery according to the above-described exemplaryembodiments may include the lithium-transition metal composite oxideparticle having a lattice strain of 0.18 or less and satisfying an XRDpeak intensity ratio of a predetermined value or less as a cathodeactive material. Thus, particle strength may be increased, and outputcharacteristics may be improved.

By controlling the lattice strain to 0.18 or less, so that a phenomenonin which particles are cracked in boundary regions between crystalgrains or between particles may be prevented. Accordingly, gasgeneration in a high-temperature environment and/or gas generationduring charging and discharging may be prevented, thus the life-spancharacteristics of the secondary battery may be improved.

In addition, a lithium diffusion distance may be shortened bycontrolling the XRD peak intensity ratio to a predetermined value orless, thus output characteristics of the battery may be improved. Inthis case, a decrease in relative life-span characteristics due to theshortening of the lithium diffusion distance may be relieved orcompensated by controlling the lattice strain to 0.18 or less.

Thus, the lithium-transition metal composite oxide particle satisfyingthe above-described lattice strain and XRD peak intensity ratio have theabove-described particle strength and output performance, and thusoperational stability, life-span characteristics, and outputcharacteristics may be improved together.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are a schematic planar view and a cross-sectional viewillustrating a lithium secondary battery according to exemplaryembodiments, respectively; and

FIGS. 3 and 4 are exemplary graphs illustrating a method of applyingWilliamson-Hall method to a numerical value acquired through X-raydiffraction (XRD) analysis in an exemplary embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide a lithium secondary batteryincluding a lithium-transition metal composite oxide particle having alattice strain value and an XRD peak intensity ratio in a predeterminedrange, respectively, as a cathode active material.

Hereinafter, the present invention will be described in detail withreference to the accompanying drawings. However, those skilled in theart will appreciate that such embodiments described with reference tothe accompanying drawings are provided to further understand the spiritof the present invention and do not limit subject matters to beprotected as disclosed in the detailed description and appended claims.

FIGS. 1 and 2 are a schematic top planar view and a schematiccross-sectional view, respectively, illustrating a lithium secondarybattery in accordance with exemplary embodiments. Hereinafter, a cathodeactive material for a lithium secondary battery and a lithium secondarybattery including the same will be described with reference to FIGS. 1and 2 .

Referring to FIGS. 1 and 2 , a lithium secondary battery may include anelectrode assembly that may include a cathode 100, an anode 130 and aseparation layer 140 interposed between the cathode 100 and the anode130. The electrode assembly may be inserted in a case 160 together withan electrolyte to be immersed therein.

The cathode 100 may include a cathode active material layer 110 formedby coating a cathode active material on a cathode current collector 105.The cathode active material may include a compound capable of reversiblyintercalating and de-intercalating lithium ions.

In exemplary embodiments, the cathode active material may includelithium-transition metal composite oxide particles.

For example, the lithium-transition metal composite oxide particle mayinclude nickel (Ni), and may further include at least one of cobalt (Co)and manganese (Mn).

For example, the lithium-transition metal composite oxide particle maybe represented by Chemical Formula 1 below.

Li_(x)Ni_(1−y)M_(y)O_(2+z)  [Chemical Formula 1]

In Chemical Formula 1, x and y may be in a range of 0.9≤x≤1.2, and0≤y≤0.7, and z may be in a range of −0.1≤z≤0.1. M may be at least oneelement selected from the group consisting of Na, Mg, Ca, Y, Ti, Zr, Hf,V, Nb, Ta, Cr, Mo, W, Mn, Co, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn andZr.

In some embodiments, a molar ratio or concentration (1−y) of Ni inChemical Formula 1 may be 0.8 or more, and exceeds 0.8 in a preferredembodiment.

Ni may serve as a transition metal related to power and capacity of alithium secondary battery. Thus, as described above, a high-Nicomposition may be applied to the lithium-transition metal compositeoxide particle so that high-power cathode and lithium secondary batterymay be provided.

However, as a content of Ni increases, long-term storage stability andlife-span stability of the cathode or the secondary battery may berelatively deteriorated. However, according to exemplary embodiments,life stability and capacity retention properties may be improved byemploying Mn while maintaining electrical conductivity by including Co.

In some embodiments, the cathode active material or thelithium-transition metal composite oxide particle may further include acoating element or a doping element. For example, the coating element orthe doping element may include Al, Ti, Ba, Zr, Si, B, Mg, P, an alloythereof or an oxide thereof. These may be used alone or in combinationthereof. The cathode active material particles may be passivated by thecoating or doping element, stability and life-span properties against apenetration of an external object may be further improved.

For example, a nickel-manganese-cobalt precursor (e.g.,nickel-cobalt-manganese hydroxide) and a lithium precursor (e.g.,lithium hydroxide or lithium carbonate) may be reacted by a wet mixingor a dry mixing, and then a reacted product may be fired to prepare thelithium-transition metal composite oxide particle.

The lithium-transition metal composite oxide particles may have a singlecrystal and/or polycrystalline structure in a crystallographic aspect.In an embodiment, the cathode active material may include a mixture or ablend of single crystal particles and polycrystalline particles.

In some embodiments, the lithium-transition metal composite oxideparticle may have a polycrystalline structure in crystallography. Inthis case, lattice strain described below may occur in a boundary regionbetween crystal grains, and thus, a cracking phenomenon in the boundaryregion may occur.

According to exemplary embodiments of the present invention, cracking inthe boundary region are decreased, so that gas generation by repeatedcharging and discharging and in a high-temperature environment may bereduced, and life-span characteristics of the secondary battery may befurther improved.

The lithium-transition metal composite oxide particle may have a form ofa single particle, primary particle or secondary particle in morphology.

FIGS. 3 and 4 are exemplary graphs illustrating a method of applyingWilliamson-Hall method to a numerical value acquired through an X-raydiffraction (XRD) analysis in an exemplary embodiment.

Referring to FIG. 3 , XRD peaks may be acquired by the XRD analysis, anda lattice strain of the lithium-transition metal composite oxideparticle may be obtained by applying Williamson-Hall method defined byEquation 1 below to the acquired peaks.

β cos θ=η sin θ+λ/D  [Equation 1]

In Equation 1, β represents full width at half maximum (FWHM) (rad) ofthe corresponding peak acquired through the XRD analysis, θ represents adiffraction angle (rad), η represents a lattice strain (dimensionlessnumber), λ represents anX-ray wavelength (Å), and D represents acrystallite size (Å).

In some embodiments, in the Equation 1 above, S may be a half-widthcorrecting a value derived from a device. In an embodiment, Si may beused as a standard material for reflecting the device-derived value. Inthis case, a half-width profile of Si over an entire 20 range may befitted, and the device-derived half-width may be expressed as a functionof 2θ.

Thereafter, a value obtained by subtracting and correcting the halfwidth value derived from the device in the corresponding 2θ obtainedfrom the above function may be used as S.

For example, the XRD analysis may be performed by using Cu-Kα rays as alight source for the dried powder of the lithium-transition metalcomposite oxide particle in a diffraction angle (20) range of 100 to120° at a scan rate of 0.0065°/step.

Referring to FIG. 4 , after measuring the full widths at half maximum ofall peaks appearing in the diffraction angle range, a slope may beacquired through linear regression analysis to calculate the latticestrain by substituting the acquired measurement values in Equation 1(Williamson-Hall method).

For example, the lattice strain may be defined as a slope of a straightline which is obtained when plotting a horizontal axis with sin θ and avertical axis with β cos θ in Equation 1 above.

In exemplary embodiments, the lattice strain may be 0.18 or less. Whenthe lattice strain is 0.18 or less, the lattice strain in the boundaryregion between crystal grains or between the particles of thelithium-transition metal composite oxide particle may be decreased.

When the lattice strain exceeds 0.18, the lattice strain of thelithium-transition metal composite oxide particle may be increased, andthus life-span characteristics such as gas generation and capacityretention rate at a high temperature may be deteriorated.

Accordingly, a cracking phenomenon of the particles mainly occurred inthe boundary region between crystal grains during a press process forforming the cathode active material layer 110 or when charging anddischarging the battery may be reduced. Thus, the particle strength ofthe lithium-transition metal composite oxide particle may be increased,and a high-density electrode may be implemented. In this case, forexample, an amount of gas generated at a high temperature may bereduced, and gas generation due to repeated charging/discharging may besuppressed. Thus, stable capacity characteristics even in a hightemperature environment may be provided and the life-spancharacteristics of the lithium secondary battery may be improved.

In some embodiments, the lattice strain may be in a range of 0.03 to0.18. When the lattice strain is 0.03 or more, a strength of thelithium-transition metal composite oxide is excessively increased, thusimpregnation property of the battery may be prevented from beingreduced. Thus, a deterioration in the output characteristics of thebattery may be prevented and the life-span characteristics may beimproved.

According to exemplary embodiments, the lithium-transition metalcomposite oxide particles may have an XRD peak intensity ratio definedby Equation 2 below of 8.9% or less.

XRD peak intensity ratio (%)=100×I(110)/{I(110)+I(003)}  [Equation 2]

In Equation 2, I(110) represents a peak intensity or a maximum height ofa (110) plane by X-ray diffraction (XRD) analysis of thelithium-transition metal composite oxide particle, and I(003) representsa peak intensity or a maximum height of a (003) plane by the XRDanalysis of the lithium-transition metal composite oxide particle.

For example, the XRD analysis may be performed using a Cu Kα ray as alight source for a dried powder of the lithium-transition metalcomposite oxide particles, in a range of diffraction angle (2θ) of 10oto 120o at a scan rate of 0.0065o/step.

In the above-described XRD peak intensity ratio range, an ionpropagation length and an ion diffusion length on the (110) planethrough which lithium ions are diffused may be reduced. Additionally,the ratio of the peak intensity relative to the (003) plane thatintersects the (110) plane may be adjusted to reflect an aspect ratio ofthe particles. Accordingly, a decrease in the output due to an increasein the lithium diffusion length or an excessive increase in the aspectratio of the particle may be prevented.

For example, a capacity degradation phenomenon caused by repeatedlycharging/discharging of the lithium secondary battery propagates fromthe (110) plane to an inside of the particles, thus the batterydegradation rate may be increased and the life-span characteristics ofthe battery may be relatively reduced when the diffusion length oflithium ions is reduced.

However, according to exemplary embodiments, the lithium-transitionmetal composite oxide having a lattice strain of 0.18 or less isemployed to increase the particle strength, so that a decrease inlife-span characteristics at a relatively high temperature due tocontrolling the XRD peak intensity ratio to 8.93 or less may be relievedor compensated.

Therefore, a high-density electrode may be implemented by reducingparticle cracks through the control of the lattice strain, a gasgeneration to increase life-span stability in a high-temperatureenvironment may be prevented, and output/capacity may be enhanced byincreasing lithium ion migration characteristics through the control ofthe XRD peak intensity ratio together.

In one embodiment, the XRD peak intensity ratio may be 4 to 8.9, andpreferably 5 to 8.9. Within the above range, it is possible to enhancethe output characteristics while maintaining the surface stability andlife-span characteristics of the lithium-transition metal compositeoxide particle.

For example, a transition metal precursor (e.g., a Ni—Co—Mn precursor)for preparing the lithium-transition metal composite oxide particle maybe prepared through a co-precipitation reaction.

The above-described transition metal precursor may be prepared through aco-precipitation reaction of metal salts. The metal salts may include anickel salt, manganese salt and a cobalt salt.

Examples of the nickel salt may include nickel sulfate, nickelhydroxide, nickel nitrate, nickel acetate, hydrates thereof, etc.Examples of the manganese salt may include manganese sulfate, manganeseacetate, hydrates thereof, etc. Examples of the cobalt salt may includecobalt sulfate, cobalt nitrate, cobalt carbonate, hydrates thereof, etc.

The metal salts may be mixed with a precipitating agent and/or achelating agent in a ratio that may satisfy a content or a concentrationratio of each metal described with reference to Chemical Formula 1 toprepare an aqueous solution. The aqueous solution may be co-precipitatedin a reactor to prepare the transition metal precursor.

The precipitating agent may include an alkaline compound such as sodiumhydroxide (NaOH), sodium carbonate (Na2CO3), etc. The chelating agentmay include, e.g., aqueous ammonia (e.g., NH3H2O), ammonium carbonate(e.g., NH3HCO3), or the like.

For example, a temperature of the co-precipitation reaction may beadjusted in a range from about 40° C. to 60° C. A reaction time may beadjusted in a range from about 24 to 72 hours.

For example, the lithium-transition metal composite oxide particle maybe prepared by reacting the transition metal precursor and a lithiumprecursor with each other. The lithium precursor compound may include,e.g., lithium carbonate, lithium nitrate, lithium acetate, lithiumoxide, lithium hydroxide, or the like. These may be used alone or incombination thereof.

Thereafter, for example, lithium impurities or unreacted precursors maybe removed through a washing process, and metal particles may be fixedor crystallinity may be increased through a heat treatment (firing)process. In an embodiment, a temperature of the heat treatment may be ina range from about 600° C. to 1000° C.

For example, the heat treatment process may include a first calcinationtreatment performed at a high temperature and a second calcinationtreatment performed at a relatively lower temperature than the hightemperature. Specifically, after the first calcination treatment isperformed, the second calcination treatment may be performed at atemperature lower than the temperature at which the first calcinationtreatment is performed. In this case, the strength and hardness of theformed lithium-transition metal composite oxide particles may beimproved, such that the life-span characteristics and driving stabilityof the secondary battery may be enhanced.

For example, the first calcination treatment may be performed at 800 to1,000° C., and the second calcination treatment may be performed at 600to 950° C.

For example, the above-described XRD peak ratio may be changed accordingto the above-described co-precipitation reaction time, the reactiontemperature, heat treatment temperature, or the like.

The cathode active material particle including the above-mentionedlithium-transition metal composite oxide particle may be mixed andstirred together with a binder, a conductive agent and/or a dispersiveagent in a solvent to form a slurry. The slurry may be coated on thecathode current collector 105, and dried and pressed to obtain thecathode 100.

The cathode current collector 105 may include stainless-steel, nickel,aluminum, titanium, copper or an alloy thereof. Preferably, aluminum oran alloy thereof may be used.

The binder may include an organic based binder such as a polyvinylidenefluoride-hexafluoropropylene copolymer (PVDF-co-HFP),polyvinylidenefluoride (PVDF), polyacrylonitrile,polymethylmethacrylate, etc., or an aqueous based binder such asstyrene-butadiene rubber (SBR) that may be used with a thickener such ascarboxymethyl cellulose (CMC).

For example, a PVDF-based binder may be used as a cathode binder. Inthis case, an amount of the binder for forming the cathode activematerial layer may be reduced, and an amount of the cathode activematerial may be relatively increased. Thus, capacity and power of thesecondary battery may be further improved.

The conductive agent may be added to facilitate an electron mobilitybetween the active material particles. For example, the conductive agentmay include a carbon-based material such as graphite, carbon black,graphene, carbon nanotube, etc., and/or a metal-based material such astin, tin oxide, titanium oxide, a perovskite material such as LaSrCoO3or LaSrMnO3.

In exemplary embodiments, the anode 130 may include an anode currentcollector 125 and an anode active material layer 120 formed by coatingan anode active material on the anode current collector 125.

The anode active material may include a material that may be capable ofadsorbing and ejecting lithium ions. For example, a carbon-basedmaterial such as a crystalline carbon, an amorphous carbon, a carboncomplex or a carbon fiber, a lithium alloy, a silicon-based compound,tin, etc., may be used. The amorphous carbon may include a hard carbon,cokes, a mesocarbon microbead (MCMB), a mesophase pitch-based carbonfiber (MPCF), etc.

The crystalline carbon may include a graphite-based material, such asnatural graphite, artificial graphite, graphitized cokes, graphitizedMCMB, graphitized MPCF, etc. The lithium alloy may further includealuminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium,or indium.

The silicon-based compound may include, e.g., silicon oxide or asilicon-carbon composite compound such as silicon carbide (SiC).

For example, the anode active material may be mixed and stirred togetherwith a binder, a conductive agent and/or a dispersive agent in a solventto form a slurry. The slurry may be coated on at least one surface ofthe anode current collector 125, and dried and pressed to obtain theanode 130.

The binder and the conductive agent substantially the same as or similarto those used in the cathode active material layer 110 may be used. Insome embodiments, the binder for the anode may include an aqueous bindersuch as such as styrene-butadiene rubber (SBR) that may be used with athickener such as carboxymethyl cellulose (CMC) so that compatibilitywith the carbon-based active material may be improved.

The separation layer 140 may be interposed between the cathode 100 andthe anode 130. The separation layer 140 may include a porous polymerfilm prepared from, e.g., a polyolefin-based polymer such as an ethylenehomopolymer, a propylene homopolymer, an ethylene/butene copolymer, anethylene/hexene copolymer, an ethylene/methacrylate copolymer, or thelike. The separation layer 140 may also be formed from a non-wovenfabric including a glass fiber with a high melting point, a polyethyleneterephthalate fiber, or the like.

In some embodiments, an area and/or a volume of the anode 130 (e.g., acontact area with the separation layer 140) may be greater than that ofthe cathode 100. Thus, lithium ions generated from the cathode 100 maybe easily transferred to the anode 130 without loss by, e.g.,precipitation or sedimentation.

In exemplary embodiments, an electrode cell may be defined by thecathode 100, the anode 130 and the separation layer 140, and a pluralityof the electrode cells may be stacked to form an electrode assemblyhaving, e.g., a jelly roll shape. For example, the electrode assembly150 may be formed by winding, laminating or folding of the separationlayer 140.

The electrode assembly 150 may be accommodated in a case 160 togetherwith an electrolyte to form the lithium secondary battery. In exemplaryembodiments, the electrolyte may include a non-aqueous electrolytesolution.

The non-aqueous electrolyte solution may include a lithium salt and anorganic solvent. The lithium salt may be represented by Li+X, and ananion of the lithium salt X may include, e.g., F—, Cl—, Br—, I—, NO3-,N(CN)2-, BF4-, C104-, PF6-, (CF3) 2PF4-, (CF3) 3PF3-, (CF3) 4PF2-, (CF3)5PF—, (CF3) 6P—, CF3SO3-, CF3CF2SO3-, (CF3SO2)2N—, (FSO2)2N—,CF3CF2(CF3)2CO—, (CF3SO2)2CH—, (SF5)3C—, (CF3SO2)3C—, CF3(CF2)7SO3-,CF3CO2-, CH3CO2-, SCN—, (CF3CF2SO2)2N—, etc.

The organic solvent may include propylene carbonate (PC), ethylenecarbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC),ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate,dimethyl sulfoxide, acetonitrile, dimethoxy ethane, diethoxy ethane,vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite,tetrahydrofuran, etc. These may be used alone or in a combinationthereof.

As illustrated in FIG. 1 , electrode tabs (a cathode tab and an anodetab) may protrude from the cathode current collector 105 and the anodecurrent collector path 125 of each electrode cell to extend to a side ofthe case 160. The electrode tabs may be fused together with the side ofthe case 160 to form an electrode lead (a cathode lead 107 and an anodelead 127) extending or exposed to an outside of the case 160.

The lithium secondary battery may be fabricated into a cylindrical shapeusing a can, a prismatic shape, a pouch shape, a coin shape, etc.

Hereinafter, preferred embodiments are proposed to more concretelydescribe the present invention. However, the following examples are onlygiven for illustrating the present invention and those skilled in therelated art will obviously understand that various alterations andmodifications are possible within the scope and spirit of the presentinvention. Such alterations and modifications are duly included in theappended claims.

Preparation of the Lithium-Transition Metal Composite Oxide Particle

NiSO₄, COSO₄ and MnSO₄ were mixed in a ratio of 0.8:0.1:0.1,respectively, using distilled water with internal dissolved oxygenremoved by bubbling with N₂ for 24 hours. The solution was introducedinto a reactor at 55° C., and a co-precipitation reaction was performedfor 36 hours using NaOH and NH₃H₂O as a precipitant and a chelatingagent to obtain Ni_(0.8)Co_(0.1)Mn_(0.1)(OH)₂ as a transition metalprecursor. The obtained precursor was dried at 80° C. for 12 hours andthen again dried at 110° C. for 12 hours.

Then, lithium hydroxide and the transition metal precursor were added ina ratio of 1.05:1 in a dry high-speed mixer, followed by uniformlystirring and mixing the same for 5 minutes. The mixture was put in acalcination furnace, heated to 950° C. at a heating rate of 2° C./min,maintained at 950° C. for 5 hours, and then naturally cooled to 900° C.and maintained for 5 hours. Oxygen was passed through continuously at aflow rate of 10 mL/min during heating and maintenance. After completionof the calcination, the mixture was naturally cooled to roomtemperature, followed by pulverizing and distributing to manufacture thelithium-transition metal composite oxide particle (Particle 1) in a formof a single particle (including single crystal and polycrystallinestructures) as a cathode active material represented byLiNi_(0.8)Co_(0.1)Mn_(0.1)O₂.

A reaction time or a reaction temperature in the reactor, or a firingtime or a firing temperature in the firing process were changed tofurther prepare particles 2 to 9. Through XRD analysis conducted on eachof the lithium-transition metal composite oxide particles, the latticestrain calculated using Equation 1 and the XRD peak intensity ratiocalculated using Equation 2 were produced and shown in Table 2 below.

Detailed XRD analysis equipment/conditions are shown in Table 1 below.

TABLE 1 XRD (X-Ray Diffractometer) EMPYREAN Maker PANalytical Anodematerial Cu K-Alpha1 wavelength 1.540598 Å Generator voltage 45 kV Tubecurrent 40 mA Scan Range 10-120° Scan Step Size 0.0065° Divergence slit¼° Antiscatter slit ½°

TABLE 2 XRD peak intensity ratio Lattice [100 × I(110)/ Classificationstrain {I(110) + I(003)}] Lithium- Particle 1 0.15 6.89 transitionParticle 2 0.14 7.42 metal Particle 3 0.15 3.9 composite Particle 4 0.199.83 oxide Particle 5 0.21 9.8 particle Particle 6 0.2 9.8 Particle 70.11 8.974 Particle 8 0.22 7.856 Particle 9 0.19 8.6

Example 1

A secondary battery was manufactured using the above-described particle1 as a cathode active material. Specifically, the cathode activematerial particle, Denka Black as a conductive additive and PVDF as abinder were mixed by a weight ratio of 97:2:1 to form a cathode slurry.The cathode slurry was coated, dried, and pressed on an aluminumsubstrate to form a cathode. A density of the cathode after the pressingwas controlled as 3.71 g/cc

An anode slurry was prepared by mixing 93 wt % of a natural graphite asan anode active material, 5 wt % of a flake type conductive additiveKS6, 1 wt % of SBR as a binder, and 1 wt % of CMC as a thickener. Theanode slurry was coated, dried, and pressed on a copper substrate toform an anode.

The cathode and the anode obtained as described above were notched by aproper size and stacked, and a separator (polyethylene, thickness: 25μm) was interposed between the cathode and the anode to form anelectrode cell. Each tab portion of the cathode and the anode waswelded. The welded cathode/separator/anode assembly was inserted in apouch, and three sides of the pouch (e.g., except for an electrolyteinjection side) were sealed. The tab portions were also included insealed portions. An electrolyte was injected through the electrolyteinjection side, and then the electrolyte injection side was also sealed.Subsequently, the above structure was impregnated for more than 12hours.

The electrolyte was prepared by dissolving 1M LiPF6 in a mixed solventof EC/EMC/DEC (25/45/30; volume ratio), and then 1 wt % of vinylenecarbonate, 0.5 wt % of 1,3-propensultone (PRS), and 0.5 wt % of lithiumbis (oxalato) borate (LiBOB) were added.

The lithium secondary battery as fabricated above was pre-charged byapplying a pre-charging current (5 A) corresponding to 0.25 C for 36minutes. After 1 hour, the battery was degased, aged for more than 24hours, and then a formation charging-discharging (charging condition ofCC—CV 0.2 C 4.2 V 0.05 C CUT-OFF, discharging condition of CC 0.2 C 2.5V CUT-OFF) was performed.

Example 2

A secondary battery was manufactured according to the same procedures asdescribed in Example 1, except that the target electrode density of thecathode was controlled to 3.85 g/cc.

Example 3

A secondary battery was manufactured according to the same procedures asdescribed in Example 1, except that the above-describe particle 2 wereused as a cathode active material.

Example 4

A secondary battery was manufactured according to the same procedures asdescribed in Example 3, except that the target electrode density of thecathode was controlled to 3.85 g/cc.

Example 5

A secondary battery was manufactured according to the same procedures asdescribed in Example 1, except that the above-described particle 3 wereused as a cathode active material.

Comparative Example 1

A secondary battery was manufactured according to the same procedures asdescribed in Example 1, except that the above-described particle 4 wereused as a cathode active material.

Comparative Example 2

A secondary battery was manufactured according to the same procedures asdescribed in Comparative Example 1, except that the target electrodedensity of the cathode was controlled to 3.85 g/cc.

Comparative Example 3

A secondary battery was manufactured according to the same procedures asdescribed in Example 1, except that the above-described particle 5 wereused as a cathode active material.

Comparative Example 4

A secondary battery was manufactured according to the same procedures asdescribed in Example 1, except that the above-described particle 6 wereused as a cathode active material.

Comparative Example 5

A secondary battery was manufactured according to the same procedures asdescribed in Example 1, except that the above-described particle 7 wereused as a cathode active material.

Comparative Example 6

A secondary battery was manufactured according to the same procedures asdescribed in Example 1, except that the above-described particle 8 wereused as a cathode active material.

Comparative Example 7

A secondary battery was manufactured according to the same procedures asdescribed in Example 1, except that the above-described particle 9 wereused as a cathode active material.

Component particles, lattice strains, XD peak intensity ratios, andelectrode densities of the above-described examples and comparativeexamples are shown in Table 3 below.

TABLE 3 Lithium- transition metal Target XRD peak composite electrodeintensity ratio oxide density Lattice [100 × I(110)/ Classificationparticle (g/cc) strain {I(110) + I(003)}] Example 1 Particle 1 3.71 0.156.89 Example 2 3.85 Example 3 Particle 2 3.71 0.14 7.42 Example 4 3.85Example 5 Particle 3 3.71 0.15 3.9 Comparative Particle 4 3.71 0.19 9.83Example 1 Comparative 3.85 Example 2 Comparative Particle 5 3.71 0.219.8 Example 3 Comparative Particle 6 3.71 0.2 9.8 Example 4 ComparativeParticle 7 3.71 0.11 8.974 Example 5 Comparative Particle 8 3.71 0.227.856 Example 6 Comparative Particle 9 3.71 0.19 8.6 Example 7

Experimental Example

(1) Measurement (Actual Measurement) of Electrode Density

The cathodes of the above-described lithium secondary batteries of theexamples and comparative examples were coated on both sides and punchedto a size of 12 pi (diameter: 12 mm), and then electrode densities weremeasured through the following equation.

Electrode density(g/cc)=(Weight of double-sided coated electrode−Weightof foil)/{(Total thickness of double-sided coated electrode−Thickness offoil)×12pi area}

(2) Measurement of Hot Gas Generation

After charging (1C 4.2 V 0.1C CUT-OFF) the above-described lithiumsecondary batteries of the examples and comparative examples, the amountof gas generated after 1 week and after 4 weeks of storage in a 60° C.constant temperature chamber was measured using a gas chromatography(GC) analysis. To measure a total amount of the generated gas, a holewas formed through the vacuum chamber having a predetermined volume (V)and a pressure change was measured to calculate a volume of thegenerated gas.

(3) Measurement of Amount of Gas Generated after Repeatedly Charging andDischarging

Charging (CC—CV 1.0 C 4.2V 0.05C CUT-OFF) and discharging (CC 1.0C 2.7VCUT-OFF) of the lithium secondary batteries of Examples and ComparativeExamples were repeated 100 times and 300 times in a chamber at 45° C.,and then the amount of gas was measured by the same method as that inthe above (1).

(4) Measurement of Life-Span (Capacity Retention Rate) at 45° C.

After charging (1C 4.2 V 0.1C CUT-OFF) the above-described lithiumsecondary batteries of the examples and comparative examples, andstoring them in a thermostatic chamber at 45° C., capacity retentionrates after 4 weeks were calculated by calculating discharge capacityafter 4 weeks as a percentage (%) compared to the initial dischargecapacity.

After repeatedly charging (CC—CV 1.0C 4.2 V 0.05C CUT-OFF) anddischarging (CC 1.0C 2.7 V CUT-OFF) the above-described lithiumsecondary batteries of the examples and the comparative examples 300times in a chamber at 45° C., capacity retention rates after 300 cycleswere evaluated by calculating the discharge capacity at 300 times as apercentage (F) compared to the discharge capacity at one time.

The evaluation results are shown in Table 4 below.

TABLE 4 Gas generation Gas generation Electrode at a high after repeatedCapacity density temperature charging/discharging retention rate (g/cc)(mL) (mL) (%) Actual After After After After After After ClassificationTarget measurement 1 week 4 weeks 100 cycles 300 cycles 4 weeks 300cycles Example 1 3.71 3.75 7.69 12.97 6.72 10.35 96.9 95 Example 2 3.853.88 8.09 16.13 6.84 11.25 95.9 94 Example 3 3.71 3.71 6.90 11.04 6.9110.92 96.5 95 Example 4 3.85 3.85 7.92 17.23 7.04 11.88 95.8 95 Example5 3.71 3.68 7.68 15.01 10.11 19.51 96.9 90 Conparative 3.71 3.66 24.932.04 20.2 24.34 96.9 83 Example 1 Conparative 3.85 3.73 15.56 19.6117.08 26.27 96.9 82 Example 2 Conparative 3.71 3.65 48.44 67.03 36.630.38 90.6 80 Example 3 Conparative 3.71 3.63 48.74 65.31 36.3 32.3191.3 81 Example 4 Conparative 3.71 3.72 22.35 30.64 18.6 22.1 95.3 87Example 5 Conparative 3.71 3.68 24.87 37.56 26.29 33.17 94.9 84 Example6 Conparative 3.71 3.6 23.2 28.1 19.5 23.3 96.9 83 Example 7

Referring to Table 4, in the case of the lithium secondary batteries ofthe examples, which use the lithium-transition metal composite oxideparticle having a lattice strain of 0.18 or less and satisfying an XRDpeak intensity ratio of 8.9% or less, good capacity retention rates wereensured with higher electrode densities while having suppressed amountof gas generated as a whole than the lithium secondary batteries of thecomparative examples.

Specifically, the lithium secondary batteries of Examples 1 to 3 havinga lattice strain of 0.03 to 0.18 and an XRD peak intensity ratio of 4 to8.9% exhibited significantly reduced amount of gas generated at a hightemperature and during repeated charging/discharging and excellentcapacity retention rate compared to the lithium secondary batteries ofthe comparative examples.

However, the lithium secondary battery of Example 5 having an XRD peakintensity ratio of less than 4%, secured a small amount of gas generatedand an excellent capacity retention rate compared to the lithiumsecondary batteries of the comparative examples, but exhibited asomewhat higher amount of gas generated and a lower capacity retentionrate than the lithium secondary batteries of Examples 1 to 3.

Comparative Example 5 is a comparative example relating to a lithiumsecondary battery having a lattice strain of 0.18 or less, and an XRDpeak intensity ratio exceeding 8.9%, and Comparative Examples 6 and 7are comparative examples relating to lithium secondary batteries havingan XRD peak intensity ratio of 8.9% or less, and a lattice strain ofexceeding 0.18. Similar to Comparative Examples 1 to 4, the lithiumsecondary batteries of Comparative Examples 5 to 7 had a large amount ofgas generated and reduced capacity retention rate as a whole compared tothe lithium secondary batteries of the examples.

DESCRIPTION OF REFERENCE NUMERALS

-   100: Cathode-   105: Cathode current collector-   107: Cathode lead-   110: Cathode active material layer-   120: Anode active material layer-   125: Anode current collector-   127: Anode lead-   130: Anode-   140: Separation membrane-   150: Electrode assembly-   160: Case

What is claimed is:
 1. A cathode active material for a lithium secondarybattery comprising a lithium-transition metal composite oxide particle,wherein the lithium-transition metal composite oxide particle containsnickel, wherein a lattice strain (n) calculated by applyingWilliamson-Hall method defined by Equation 1 to XRD peaks measuredthrough X-ray diffraction (XRD) analysis is 0.18 or less:β cos θ=η sin θ+λ/D  [Equation 1] wherein, in Equation 1, β representsfull width at half maximum (FWHM) (rad) of the corresponding peakacquired through the XRD analysis, θ represents a diffraction angle(rad), η represents a lattice strain (dimensionless number), λrepresents an X-ray wavelength (Å), and D represents a crystallite size(Å).
 2. The cathode active material for a lithium secondary batteryaccording to claim 1, wherein the lattice strain of thelithium-transition metal composite oxide particle is a slope of astraight line, and the straight line is obtained by acquiring fullwidths at half maximum of all peaks appearing through the XRD analysis,and substituting the acquired full widths at half maximum in Equation 1when plotting a horizontal axis with sine and a vertical axis with β cosθ.
 3. The cathode active material for a lithium secondary batteryaccording to claim 1, wherein an XRD peak intensity ratio defined byEquation 2 is 8.9% or less:XRD peak intensity ratio (%)=100×I(110)/{I(110)+I(003)}  [Equation 2]wherein, in Equation 2, I(110) represents a maximum height of the peakof a (110) plane of the lithium-transition metal composite oxideparticle by the XRD analysis, and I(003) represents a maximum height ofthe peak of a (003) plane of the lithium-transition metal compositeoxide particle by the XRD analysis.
 4. The cathode active material for alithium secondary battery according to claim 3, wherein the XRD peakintensity ratio of the lithium-transition metal composite oxide particleis in a range from 4 to 8.9%.
 5. The cathode active material for alithium secondary battery according to claim 1, wherein thelithium-transition metal composite oxide particle has a polycrystallinestructure in crystallography.
 6. The cathode active material for alithium secondary battery according to claim 1, wherein thelithium-transition metal composite oxide particle has at least one of asingle particle, a primary particle, and a secondary particle inmorphology.
 7. The cathode active material for a lithium secondarybattery according to claim 1, wherein the lithium-transition metalcomposite oxide particle has a composition represented by ChemicalFormula 1 below:Li_(x)Ni_(1−y)M_(y)O_(2+z)  [Chemical Formula 1] (in Chemical Formula 1,x and y are in a range of 0.9≤×≤1.2, and 0≤y≤0.7, and z is in a range of−0.1≤z≤0.1, M is at least one element selected from the group consistingof Na, Mg, Ca, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Co, Fe, Cu, Ag,Zn, B, Al, Ga, C, Si, Sn and Zr).
 8. The cathode active material for alithium secondary battery according to claim 7, wherein in ChemicalFormula 1, 0≤y≤0.2.
 9. The cathode active material for a lithiumsecondary battery according to claim 7, wherein in Chemical Formula 1, Mincludes Co and Mn.
 10. The cathode active material for a lithiumsecondary battery according to claim 7, wherein the lithium-transitionmetal composite oxide particle is manufactured by reacting a transitionmetal precursor with a lithium precursor.
 11. The cathode activematerial for a lithium secondary battery according to claim 10, whereinthe transition metal precursor is a Ni—Co—Mn precursor.
 12. The cathodeactive material for a lithium secondary battery according to claim 10,wherein the lithium precursor includes lithium carbonate, lithiumnitrate, lithium acetate, lithium oxide or lithium hydroxide.
 13. Alithium secondary battery comprising: a cathode which comprises acathode active material layer comprising the cathode active materialaccording to claim 1; and an anode facing the cathode.